Applications of momentum-space similarity.

Momentum-space similarity indices were used in studies linking chemical structure to observed activity. These included (a) the biological activity of various molecules that are of interest due to their capacity for HIV inhibition; and (b) the hyperpolarisabilities of series of conjugated molecules. Study (a) included comparisons of the total valence densities of different molecules or the densities associated with particular molecular fragments. Study (b) involved, for each molecule, a comparison of the momentum-space densities of the highest occupied (HOMO) and lowest unoccupied (LUMO) molecular orbitals. The momentum-space approach, which is most sensitive to features of the long-range valence electron density, turned out to be particularly useful for cases such as these, in which the physical property or biological activity has no obvious dependence on the bonding topology of the molecules.